A) Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a gain-coupled distributed feedback type semiconductor laser device having a diffraction grating formed in a strained multi-quantum-well structure.
B) Description of the Related Art
Recent rapid spread of the Internet and information processing technology is considerably increasing the capacity of information communications. It is urgent that an optical communication network of wavelength division multiplexing (WDM) suitable for a large communication capacity is configured. A distributed feedback (DFB) laser used for a light source in WDM is required to have high speed and low cost, to say nothing of single wavelength and high output operation. In order to realize low cost, it is preferable to monolithically integrate optical components such as a semiconductor laser device, an optical modulator, an optical amplifier and an optical multiplexer/demultiplexer. A semiconductor laser device used in an optical apparatus with monolithically integrated optical components is required to have a high tolerance to back reflected light.
A λ/4 phase shift DFB laser device is in practical use as a semiconductor laser excellent in single wavelength. This laser device is of a refractive-index-coupled type and has the structure that the phase of a diffraction grating formed near a substrate or active layer is shifted by π (corresponding to a quarter wavelength in propagation medium). An antireflection film is formed on both end faces of the optical resonator so that stable single wavelength oscillation can be realized. However, since the antireflection film is formed on both end faces, it is difficult to obtain high output.
A gain-coupled distributed feedback laser device (gain-coupled DFB laser device) has been paid attention, which periodically changes the gain in the direction of light propagation by forming a diffraction grating in the active layer itself having a strained multi-quantum well structure. In this laser device, some regions of the active layer in the depth direction are periodically removed along the direction of light propagation and the remaining convex regions are periodically disposed to form the diffraction grating.
In this laser device, since antinodes of a standing wave are positioned at large gain regions (convex regions of the active layer), oscillation can be reliably made at a single wavelength without shifting the phase of the diffraction grating. This laser device is suitable for high output because it can oscillate at a single wavelength even if one end face of the optical resonator is coated with material having a high reflectivity. It has been experimentally confirmed that the characteristics of a gain-coupled DFB laser device changes are less influenced by back reflected light. Gain-coupled DFB laser devices are expected as laser devices excellent in single wavelength, high output operation and tolerance to back reflected light.
It has been reported that in conventional gain-coupled DFB laser devices, the wavelength of spontaneous emission or photoluminescence (PL) in a convex region of an active layer formed with a diffraction grating shifts (blue-shifts) to the shorter wavelength side by about several tens meV from the wavelength of the active layer before the diffraction grating is formed.